This invention relates generally to nuclear reactors and more particularly, to apparatus and methods for repairing piping within reactor pressure vessels of such reactors.
A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A core shroud, or shroud, typically surrounds the core and is supported by a shroud support structure.
Boiling water reactors have numerous piping systems, and such piping systems are utilized, for example, to transport water throughout the RPV. For example, core spray piping is used to deliver water from outside the RPV to core spray spargers inside the RPV. The core spray piping and spargers deliver water flow to the reactor core.
Intergranular stress corrosion cracking (IGSCC) is a known phenomenon occurring in reactor components, such as structural members, piping, fasteners, and welds, exposed to high temperature water. The reactor components are subject to a variety of stresses associated with, for example, differences in thermal expansion, the operating pressure needed for the containment of the reactor cooling water, and other sources such as residual stresses from welding, cold working and other inhomogeneous metal treatments. In addition, water chemistry, welding, heat treatment and radiation can increase the susceptibility of metal in a component to IGSCC.
Conditions exist in the reactor which contribute to IGSCC of the core spray piping. One area of susceptibility in the core spray piping is the welded joints between the sparger T-box and its associated distribution headers. The sparger T-box is the junction where the core spray downcomer supply pipe penetrates the shroud branches to distribution sparger pipes. The core spray system prevents excessive fuel clad temperature in the event of a Loss of Coolant Accident (LOCA) by delivering cooling water to the core region of the reactor. In the event that through-wall circumferential cracking should occur at these welded joints, the system may be compromised.
In order to prevent unacceptable leakage and to ensure that the core spray system delivers the necessary volumetric flow rate to the reactor core, it would be desirable to provide a clamping system to provide structural integrity to the sparger T-box and to hold the welded joints together in the event that one or more welds fail.
A core spray sparger T-box attachment assembly for a nuclear reactor vessel includes a downcomer pipe coupling and a sparger T-box clamp. The pressure vessel includes a shroud, a sparger T-box penetrating the shroud, a plurality of sparger distribution header pipes coupled to the sparger T-box, and a downcomer pipe. The sparger header pipes include at least one sparger nozzle.
The downcomer pipe coupling includes a cylindrical outer housing, a center portion, and a plurality of vanes extending from an inner surface of the outer housing to the center portion. The outer housing is coupled to a downcomer pipe at one end, and includes a flange extending from the opposite end. The center portion includes a threaded axial bore therethrough. A draw bolt threadedly engages the axial bore of the center portion, and connects the downcomer pipe coupling to the sparger T-box clamp.
The sparger T-box clamp includes an anchor having a draw bolt opening, and a plurality of legs extending from a face of the anchor. The legs are configured to engage an inside surface of the shroud and are machined or trimmed so that the anchor face is parallel to the exterior surface of the sparger T-box. A first and a second clamp block are connected to opposite sides of the anchor with dove-tail joints. The clamp blocks are positioned to be substantially aligned with one another. Each clamp block includes a threaded stop bolt opening extending therethrough. A stop bolt extends through each stop bolt opening. Each stop bolt has a conical shaped distal end which is sized to mate with a conical shaped opening machined in the sparger distribution header pipes.
A first and a second clamping element are connected to the first clamp block and second clamp blocks by a plurality of clamp bolts. Also, a first and a second stop bolt extending through said stop bolt opening of said first and said second clamp block respectively.
A seal plate is coupled to the anchor with a plurality of adjusting screws. The seal plate includes a plurality of adjusting screw openings and a draw bolt opening sized to receive the draw bolt. The anchor include a plurality of threaded adjusting screw openings sized to recieve the adjusting screws.
The above described core spray sparger T-box attachment assembly mechanically couples the downcomer pipe to the shroud and sparger T-box. Also, the above described core spray sparger T-box attachment assembly provides a clamping system to provide structural integrity to the sparger T-box and to hold the welded joints together in the event that one or more welds fail.